The present invention relates to an automatic transmission for a vehicle in which multiple speeds (or multiple gear stages) or a wide gear ratio range are required.
Conventionally, there have been proposed various multiple-speed automatic transmissions that realize eight forward speeds using three planetary gearsets and six friction elements. For example, Japanese Patent Application Unexamined Publication No. 2001-182785 discloses such an eight forward-speed transmission that includes a double-pinion planetary gearset, a Ravigneaux planetary gear unit (i.e., a combination of a double-pinion planetary gearset and a single-pinion planetary gearset), four clutches, and two brakes.
However, in the automatic transmission of the above reference example which realizes eight forward speeds using three planetary gearsets and six friction elements, the following problems are caused due to the two double-pinion planetary gearsets used in the automatic transmission.
(1) A diameter of the pinion gears is reduced to thereby lower endurance reliability of the transmission.
(2) The number of parts in the transmission is increased to thereby cause increase in cost.